In recent years, an Optical Transport Network (OTN) has been standardized at ITU-T. OTN is based on the premise of wavelength-division multiplexing (WDM), with which the significant increase in Internet traffic can be accommodated. OTN has been standardized as a platform for performing transmission in a transparent manner. Specifically, in OTN transmission, an upper-level layer can totally disregard the lower-level layer, when transmitting client signals in end-to-end communication. Examples of client signals are those of a synchronous network such as Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH), as well as those of an asynchronous network such as Internet Protocol (IP) or Ethernet (registered trademark). The interface and the frame format of OTN have been standardized as the ITU-T standard G.709, and are rapidly being installed in commercial systems. OTN uses an Optical channel Data Unit (ODU), in which overhead (OH) bytes used for connection and quality control are attached to the payload area of an Optical channel Payload Unit (OPU).
FIG. 1 illustrates an example of a conventional cross-connect system in which both SONET and ODU are used. First, a description is given of a flow of signals of SONET frames. SONET frames are input from terminals 1-1 through 1-n. Serial/parallel conversion is performed at high-speed interface (inf) units 2-1 through 2-n on the SONET input side. Accordingly, the speed is reduced in units of cross-connection processing (for example, 311.04 Mb/s×8 lines of parallel signals in the case of inputting 2.4 G). Next, low-speed inf units 3-1-1 through 3-n-m perform synchronization detection of SONET signals, transfer the SONET signals onto clock signals of the device, and align front positions of data for cross-connection performed in units of columns. The process of aligning the front positions of data is performed to facilitate the functions of interchanging channels and interchanging time slots in units of columns at a SONET cross-connect unit 4 at the subsequent stage.
At the SONET cross-connect unit 4, the data items whose front positions have been aligned are input. The SONET cross-connect unit 4 includes a space switch 4a and a time switch 4b. For example, a signal input from the terminal 1-1 (DATA IN 1) is switched, at the space switch 4a, to line 2 (DATA IN 2) according to a data switch control signal based on an output data switch request signal from a CPU 10, and the data is simultaneously latched. The latched data is switched once again by a data switch control signal at the time switch 4b at a time when all data items corresponding to one column are ready.
The data that is switched at the SONET cross-connect unit 4 is supplied to low-speed inf units 5-1-1 through 5-n-m, and SONET frames are regenerated. At high-speed inf units 6-1 through 6-n on the SONET output side, the SONET frames are parallel/serial converted to have the same SONET frame format as when the SONET frames had first been input to the terminals 1-1 through 1-n. The SONET frames are then output from the terminals 8-1 through 8-n through selectors (SEL) 7-1 through 7-n. 
Next, a description is given of a flow of signals in ODU frames. ODU frames are input from the terminals 1-1 through 1-n, and are converted into ODU intermediate frames described below (164.42 Mb/s×40 parallel in the case of inputting 6.577 G), at high-speed inf units 12-1 through 12-n on the ODU input side and intermediate frame mapping units 13-1-1 through 13-n-k. The ODU intermediate frames are input to an ODU cross-connect unit 14. The ODU cross-connect unit 14 switches the ODU intermediate frames according to data switch signals based on output data switch request signals from the CPU 10. The ODU intermediate frames are switched only by a channel space switch 14a. The ODU cross-connect unit 14 does not have a time switch for interchanging time slots.
The data items switched at the ODU cross-connect unit 14 are ODU intermediate frames. Therefore, ODU frames are regenerated at intermediate frame demapping units 15-1-1 through 15-n-k. At high-speed inf units 16-1 through 16-n on the ODU output side, the ODU frames are parallel/serial converted to have the same ODU frame format as that when the ODU frames had first been input to the terminals 1-1 through 1-n. The ODU frames are then output from the terminals 8-1 through 8-n through selectors (SEL) 7-1 through 7-n. 
As described above, in a conventional cross-connect system in which both SONET and ODU are used, SONET and ODU can coexist by having cross-connect units provided for the respective types of input signals.
Incidentally, the following technology of controlling and managing devices and networks is known. Specifically, an OTN cross-connect device constituting a second network is located at the boundary between a first network and the second network. In OTN frames of client interface cards installed in an OTN cross-connect device, SDH/SONET signals are stored in an incoherent manner. By using the overhead of OTN frames in the second network, devices and networks are controlled and managed (see, for example, patent document 1).
Patent document 1: Japanese Laid-Open Patent Publication No. 2003-188919
In a cross-connect system in which both SONET and ODU are used, SONET and ODU use different cross-connect units. In the case of SONET, the cross-connect unit is a TDM (Time Division Multiplexing) switch including a space switch and a time switch. In the case of ODU, the cross-connect unit includes only a space switch. SONET and ODU use different cross-connect units to implement the cross-connect function.
Furthermore, when SONET and ODU coexist in the same system, only one of the two formats is used selectively, and therefore an idle circuit exits. That is to say, the cross-connection functions are provided separately for SONET and ODU, but only one function is used at a time.